Sin is the name of the moon-god in Babylonia and Assyria also known as Nannar, the “illuminer”. The two chief seats of his worship were Ur in the south and Harran considerably to the north, but the cult at an early period spread to other centres, and temples of the moon-god are found in all the large cities in Babylonia and Assyria. He is commonly designated as Ez-nu, “Lord of Wisdom”, and this attribute clings to him through out all periods. During the period (c. 2600-2400 B.C.) that Ur exercised a large measure of supremacy over the Euphrates valley, Sin was naturally regarded as the head of the pantheon.

We are justified in supposing that the cult of the moon-god was brought into Babylonia by the Semitic nomads from Arabia. The moon-god is par excelence the god of the nomadic peoples, their guide and protector at night when, during a great part of the year, they undertake their wanderings, just as the sun-god is the chief god of an agricultural people. The cult once introduced would tend to persevere, and the development of astrological science culminating in a calendar and in a system if interpretation of the movements and occurrences in the starry heavens would be an important factor in maintaining the position of Sin in the pantheon. The name of Sin's chief sanctuary at Ur was E-gish-shir-gal, “House of the great light” ; that at Harran was known as E-khul-khul, “Hous of joys”.

On seal-cylinders he is represented as an old man with flowing beard, with the crescent as his symbol. In the astral-theological system he is represented by the number 30, and the planet Venus as his daughter by the number 15. The number 30 stands obviously in connection with the thirty days as the average extent of his course until he stands again in conjunction with the sun.

The “wisdom” personified by the moon-god is likewise an expression of the science of astrology in which the observation of the moon's phases is so important a factor. The tendency to centralize the powers of the universe leads to the establishment of a doctrine of a triad consisting of Sin, Shamash and Ishtar, personifying the moon, sun and the earth as the life force.